1. Field of the Invention
The present invention relates to an outer-rotor type spindle motor used to drive a disk such as magnetic disks and optical disks of recording units wherein data is stored on the disks.
2. Description of the Prior Art
In general, an outer-rotor type spindle motor is used as a drive motor of a recording unit wherein data is stored on the disk. In this type of spindle motor: a head portion (i.e., a front-end portion) of a spindle firmly fitted in a center of a rotor yoke is projected; and the spindle is adapted to rotate together with the rotor yoke.
The problem of reducing the office automation instruments (such as personal computers and word processors) in size and thickness has been a matter of increasing importance, which also makes it necessary to reduce in size and thickness a drive mechanism of the recording unit used in the office automation instrument.
In such drive mechanism of the recording unit in the office automation instrument, a spindle of a drive motor for the recording unit has its head portion. projected from a rotor yoke which is supported by the spindle. Consequently, it is possible to reduce the drive motor in thickness when a projected portion of the spindle is reduced in length. This is, however, practically impossible for the following reasons.
That is, in the recording unit, a suitable media, for example such as a 3.5-inch floppy disk, is fitted to the spindle of the motor by means of an ejector of a loading mechanism so as to be loaded onto a disk chucking mount. In this loading operation, the media or disk is horizontally moved to the vicinity of the spindle, and then moved downward in a direction inclining a very fine angle when the disk reaches a position immediately in front of the spindle, which enables the disk to have its mounting hole fitted to the head portion of the spindle.
As for the shape of the spindle, since the disk enters the loading mechanism of the recording unit in a fixed direction, it is necessary to form the head portion of the spindle into a hemispherical shape which makes it possible to load the disk onto the spindle in any angular position in which the spindle stops to stay. On the other hand, as shown in FIG. 10, a spindle motor for driving the disk of a magnetic recording unit has a construction in which: a flange 6a extending downward is formed in an outer peripheral portion of a rotor yoke 6; a plurality of permanent magnets 5, i.e., N's and S's are so arranged as to be spaced alternately with each other on an inner surface of the flange 6a; the rotor yoke 6 has its central portion firmly fitted to a spindle 4 of the motor; a ball bearing 21 has a plurality of balls interposed between an outer ring 21b and an inner ring 21a, and has its inner ring 21b fitted to the spindle 4; a stator holder 2, which also acts as a bearing housing, is mounted on the outer ring 21b or outer peripheral surface of the outer ring 21b; and, a stator 3 is firmly fitted to an outer peripheral surface of the stator holder 2, and has a plurality of soft pole plates 3a which radially extend, on each of which plates 3a a solenoid 3b is mounted. The stator holder 2 is fixedly mounted on a board 1. Upon energizing of the motor, the spindle 4 is rotatably driven together with the rotor yoke 6.
As described above, in the motor having the above construction, the bearing 21 is interposed between the spindle 4 and the stator 3. In case of the ball bearing which includes the inner and the outer ring, as shown in FIG. 11, since shielding plates 15 are provided between the inner and the outer ring so as to shield opposite sides of the balls from dust, the bearing 21 is large in thickness, i.e, large in a longitudinal direction of the spindle 4.
In the motor, it is required to prevent the spindle from vibrating in stabilizing the motor in operation, which is accomplished by providing a pair of ball bearings in series on the spindle. However, in case of the conventional shielded ball bearings, since even one of these conventional bearings is considerably large in thickness, it is impossible to provide these conventional bearings in series on the spindle, because they become quite large in thickness and make it impossible to reduce the motor in thickness.
In order to solve the above problem, heretofore proposed are: a construction shown in FIG. 10, in which a ball bearing 21 and an oil-impregnated metal ring 16 are arranged in series on the spindle; and another construction shown in FIG. 12, in which the ball bearing is not used and only the metal ring 16 is arranged on the spindle to reduce the motor in thickness. In any of these proposed constructions, since the metal ring 16 is of a sliding type producing a relatively large frictional resistance in comparison with the ball bearing, the motor using such metal ring tends to: increase its electric consumption; reduce its service life due to wearing of the ring metal; and be inferior in reliability to a motor using the ball bearing. These are disadvantages inherent in the conventional motor using the metal ring.
In order to reduce the motor in thickness, it is necessary to use as thin a ball bearing (a thickness of which is denoted by the reference character "H" shown in FIG. 11) as possible. However, the ball bearing having a small thickness H naturally requires small-diameter balls. As a result, such ball bearing suffers from:
(1) a small rate load; and
(2) a large frictional resistance, which causes a large loss of torque and increases the electric consumption.